Calcium sulfate
Names | |
---|---|
Other names
Sulfate of lime
Drierite Gypsum | |
Identifiers | |
| |
3D model (
JSmol ) |
|
ChEBI |
|
ChEMBL | |
ChemSpider | |
DrugBank | |
ECHA InfoCard
|
100.029.000 |
EC Number |
|
E number | E516 (acidity regulators, ...) |
7487 | |
KEGG | |
PubChem CID
|
|
RTECS number
|
|
UNII |
|
CompTox Dashboard (EPA)
|
|
| |
| |
Properties | |
CaSO4 | |
Molar mass | 136.14 g/mol (anhydrous) 145.15 g/mol (hemihydrate) 172.172 g/mol (dihydrate) |
Appearance | white solid |
Odor | odorless |
Density | 2.96 g/cm3 (anhydrous) 2.32 g/cm3 (dihydrate) |
Melting point | 1,460 °C (2,660 °F; 1,730 K) (anhydrous) |
0.26 g/100ml at 25 °C (dihydrate)[1] | |
Solubility product (Ksp)
|
4.93 × 10−5 mol2L−2 (anhydrous) 3.14 × 10−5 (dihydrate) [2] |
Solubility in glycerol | slightly soluble (dihydrate) |
Acidity (pKa) | 10.4 (anhydrous) 7.3 (dihydrate) |
-49.7·10−6 cm3/mol | |
Structure | |
orthorhombic
| |
Thermochemistry | |
Std molar
entropy (S⦵298) |
107 J·mol−1·K−1 [3] |
Std enthalpy of (ΔfH⦵298)formation |
-1433 kJ/mol[3] |
Hazards | |
NFPA 704 (fire diamond) | |
Flash point | Non-flammable |
NIOSH (US health exposure limits): | |
PEL (Permissible)
|
TWA 15 mg/m3 (total) TWA 5 mg/m3 (resp) [for anhydrous form only][4] |
REL (Recommended)
|
TWA 10 mg/m3 (total) TWA 5 mg/m3 (resp) [anhydrous only][4] |
IDLH (Immediate danger) |
N.D.[4] |
Safety data sheet (SDS) | ICSC 1589 |
Related compounds | |
Other cations
|
Magnesium sulfate Strontium sulfate Barium sulfate |
Related desiccants
|
Calcium chloride Magnesium sulfate |
Related compounds
|
Plaster of Paris
Gypsum |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|
Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related
Hydration states and crystallographic structures
The compound exists in three levels of hydration corresponding to different crystallographic structures and to minerals:
- CaSO
4 (zirconium orthosilicate(zircon): Ca2+
is 8-coordinate, SO2−
4 is tetrahedral, O is 3-coordinate. - CaSO
4·2H
2O (gypsum and selenite (mineral)): dihydrate.[7] - CaSO
4·1/2H
2O (plaster of Paris. Specific hemihydrates are sometimes distinguished: α-hemihydrate and β-hemihydrate.[8]
Uses
The main use of calcium sulfate is to produce plaster of Paris and stucco. These applications exploit the fact that calcium sulfate which has been powdered and calcined forms a moldable paste upon hydration and hardens as crystalline calcium sulfate dihydrate. It is also convenient that calcium sulfate is poorly soluble in water and does not readily dissolve in contact with water after its solidification.
Hydration and dehydration reactions
With judicious heating, gypsum converts to the partially dehydrated mineral called
- CaSO4 · 2 H2O → CaSO4 · 1/2 H2O + 1+1/2 H2O↑
The
- CaSO4 · 1/2 H2O + 1+1/2 H2O → CaSO4 · 2 H2O
This reaction is
On heating to 180 °C (356 °F), the nearly water-free form, called γ-anhydrite (CaSO4·nH2O where n = 0 to 0.05) is produced. γ-Anhydrite reacts slowly with water to return to the dihydrate state, a property exploited in some commercial desiccants. On heating above 250 °C, the completely anhydrous form called β-anhydrite or "natural" anhydrite is formed. Natural anhydrite does not react with water, even over geological timescales, unless very finely ground.
The variable composition of the hemihydrate and γ-anhydrite, and their easy inter-conversion, is due to their nearly identical crystal structures containing "channels" that can accommodate variable amounts of water, or other small molecules such as methanol.
Food industry
The calcium sulfate hydrates are used as a coagulant in products such as tofu.[9]
For the
It is known in the E number series as E516, and the UN's FAO knows it as a firming agent, a flour treatment agent, a sequestrant, and a leavening agent.[10]
Dentistry
Calcium sulfate has a long history of use in dentistry.[11] It has been used in bone regeneration as a graft material and graft binder (or extender) and as a barrier in guided bone tissue regeneration. It is a biocompatible material and is completely resorbed following implantation.[12] It does not evoke a significant host response and creates a calcium-rich milieu in the area of implantation.[13]
Other uses
When sold at the anhydrous state as a desiccant with a color-indicating agent under the name
Up to the 1970s, commercial quantities of
- 2 CaSO4 + 2 SiO2 + C → 2 CaSiO3 + 2 SO2 + CO2[15]
The plants made sulfuric acid by the “Anhydrite Process”, in which
CaSO4 + 2 C → CaS + 2CO2
3 CaSO4 + CaS + 2 SiO2 → 2 Ca2SiO4 (belite) + 4 SO2
3 CaSO4 + CaS → 4 CaO + 4 SO2
Ca2SiO4 + CaO → Ca3OSiO4 (alite)
2 SO2 + O2 → 2 SO3 (in the presence of the
SO3 + H2O → H2SO4 [14]
Because of its use in an expanding niche market, the plant at Whitehaven, Cumbria continued to expand in a manner not shared by the other Anhydrite Process plants. The anhydrite mine opened on 11/1/1955, and the acid plant started on 14/11/1955. For a while in the early 1970s, it became the largest sulfuric acid plant in the UK, making about 13% of national production, and it was by far the largest Anhydrite Process plant ever built.[16]
Production and occurrence
The main sources of calcium sulfate are naturally occurring gypsum and anhydrite, which occur at many locations worldwide as evaporites. These may be extracted by open-cast quarrying or by deep mining. World production of natural gypsum is around 127 million tonnes per annum.[17]
In addition to natural sources, calcium sulfate is produced as a by-product in a number of processes:
- In
- SO2 + 0.5 O2 + CaCO3 → CaSO4 + CO2
Related sulfur-trapping methods use lime and some produces an impure calcium sulfite, which oxidizes on storage to calcium sulfate.
- In the production of phosphoric acid from phosphate rock, calcium phosphate is treated with sulfuric acid and calcium sulfate precipitates. The product, called phosphogypsum is often contaminated with impurities making its use uneconomic.
- In the production of hydrogen fluoride, calcium fluoride is treated with sulfuric acid, precipitating calcium sulfate.
- In the refining of zinc, solutions of zinc sulfate are treated with hydrated lime to co-precipitate heavy metals such as barium.
- Calcium sulfate can also be recovered and re-used from scrap drywall at construction sites.
These precipitation processes tend to concentrate radioactive elements in the calcium sulfate product. This issue is particular with the phosphate by-product, since phosphate ores naturally contain
Calcium sulfate is also a common component of fouling deposits in industrial heat exchangers, because its solubility decreases with increasing temperature (see the specific section on the retrograde solubility).
Retrograde solubility
The dissolution of the different crystalline phases of calcium sulfate in water is
The retrograde solubility of calcium sulfate is also responsible for its precipitation in the hottest zone of heating systems and for its contribution to the formation of scale in boilers along with the precipitation of calcium carbonate whose solubility also decreases when CO2 degasses from hot water or can escape out of the system.
On planet Mars
2011 findings by the Opportunity rover on the planet Mars show a form of calcium sulfate in a vein on the surface. Images suggest the mineral is gypsum.[22]
See also
- Calcium sulfate (data page)
- Alabaster
- Anhydrite
- Bathybius haeckelii
- Chalk (calcium carbonate)
- Gypsum
- Gypsum plaster
- Phosphogypsum
- Selenite (mineral)
- Flue-gas desulfurization
References
- S2CID 132916752.
- ^ D.R. Linde (ed.) "CRC Handbook of Chemistry and Physics", 83rd Edition, CRC Press, 2002
- ^ ISBN 978-0-618-94690-7.
- ^ a b c NIOSH Pocket Guide to Chemical Hazards. "#0095". National Institute for Occupational Safety and Health (NIOSH).
- .
- .
- ^ ISBN 0-12-683900-X, pp. 186-187.
- ^ "About tofu coagulant". www.soymilkmaker.com. Sanlinx Inc. 31 August 2015. Archived from the original on 14 March 2015. Retrieved 10 January 2008.
- ^ a b "Compound Summary for CID 24497 - Calcium Sulfate". PubChem.
- ISSN 0032-5791.
- PMID 16393128.
- ^ "Biphasic Calcium Sulfate - Overview". Augma Biomaterials. 2020-03-25. Retrieved 2020-07-16.
- ^ a b Anhydrite Process
- ^ COMMONWEALTH OF AUSTRALIA. DEPARTMENT OF SUPPLY AND SHIPPING. BUREAU OF MINERAL RESOURCES GEOLOGY AND GEOPHYSICS. REPORT NO.1949/44 (Geol. Ser. No. 27) by E.K. Sturmfels THE PRODUCTION OF SULPHURIC ACID AND PORTLAND CEMENT FROM CALCIUM SULPHATE AND ALUMINIUM SILICATES
- ^ Whitehaven Cement Plant
- ^ Gypsum, USGS, 2008
- ISBN 9780471484943.
- OSTI 6654998.
- ^ "Uranium from Phosphates | Phosphorite Uranium - World Nuclear Association".
- ^ "Brazil plans uranium-phosphate extraction plant in Santa Quitéria : Uranium & Fuel - World Nuclear News".
- ^ "NASA Mars Opportunity rover finds mineral vein deposited by water". NASA Jet Propulsion Laboratory. December 7, 2011. Retrieved April 23, 2013.